1. Field of the Invention
This invention relates to electromagnetic (EM) flow couplers and, in particular, to such couplers as adapted for regulating the flow rate or the pressure of the driven flow of a metal fluid.
2. Description of the Prior Art
As is well known in the art, EM pumps produce a pressure differential or a pressure head between the inlet and the outlet through the interaction of an electrical current and a crossed magnetic field. This interaction produces an electromagnetic body force throughout the volume of the fluid within the pump region wherever both the current density and the magnetic field are non-zero. At each such point, this body force is dependent on not only to the magnitude of the current density and magnetic flux density vectors, but also to their relative orientation. The maximum force density and resulting pressure differential occurs when the current and the magnetic field are mutually perpendicular to each other and to the direction of fluid flow.
There are two basic types of EM pumps. A first type known as an AC (alternating current) pump includes means for applying an alternating magnetic field to the liquid metal. In an AC EM pump, the alternating magnetic field induces a corresponding AC current through the liquid metal, whereby a force is exerted thereon. In a DC EM pump, a steady state electromagnetic field and a steady state or DC current are applied to the liquid metal, whereby a corresponding force is exerted thereon.
Typically, DC EM pumps are constructed in a rectangular duct by mounting two electrodes flush with the opposite side walls of the duct and placing the other two walls between magnetic pole faces. When the two electrodes are connected to an external power supply, current flows across the duct and interacts with the magnetic field to produce the axially directed body force and pressure difference along the duct. The pump's inlet and exit regions are defined roughly by the electrode edges. These regions may vary somewhat depending upon the relative location of the magnetic pole face edges. In an ideal pump, all the current would be confined to the duct volume enclosed by the electrodes and the pole faces, where the force density is the greatest. In an actual pump, however, some current leaks into the magnetic fringe region both upstream and downstream from the electrode edges. This tends to lower pump efficiency.
DC EM pumps, in small sizes up to several horsepower, have been used for many years in liquid metal cooled reactors where extremely high reliability of the pump has been required. Large, several thousand horsepower, DC EM pumps, have not been used in reactor systems in two main reasons: (1) obtaining suitable high current/low voltage power supplies, and (2) transmitting the high current from the power supply to the pump without high resistance loses. Suitable high current/low voltage power supplies may take the form of a Faraday disc generator, which generates high current at relatively low voltages as would be suitable to drive DC EM pumps. However, current transmission problems associated with such current generators require the use of large current buses. To overcome both of these problems, two ducts may be arranged side-by-side in a common magnetic field with one such duct acting as an EM generator and the other acting as an EM pump. This arrangement of EM pumps is commonly referred to as a "flow coupler" and is described in U.S. Pat. No. 2,715,190 of Brill and U.K. patent No. 745,460 of Pulley.
In a typical flow coupler, a liquid metal is caused to flow through a generator duct. Passage of the fluid through the common magnetic field generates a large current in the generator duct which is transferred to a pump duct by short, low resistance electrodes. Interaction of the current in the pump duct with the common magnetic field produces a driven flow in the pump duct. In this manner, the flow of a first liquid metal in the generator duct is "coupled" to the flow of a second liquid metal in the pump duct. The local generation of the current enables lower voltages and higher currents to be used than would be possible with an external power supply. The lower voltages, in turn, reduce end current losses and permit higher overall efficiencies, on the order of 60%, to be attained.
Early in the development of the liquid-metal fast breeder reactor (LMFBR), it was recognized that liquid metals could be pumped by the EM pumps as described above. Such EM pumps and flow couplers offer significant advantages in the reactor environment due to their inherent simplicity and lack of moving parts. In "Sodium Electrotechnology at the Risley Nuclear Power Development Laboratories," by D. F. Davidson et al., NUCLEAR ENERGY, 1981, Vol. 20, February, No. 1, pp. 79-90, there is discussed the use of EM pumps and flow couplers in LMFBR systems. EM flow couplers serve to transfer hydraulic power from one liquid metal flow circuit to another. Each circuit is isolated from the other so that there is no mixing of the two liquid metals.
The flow coupler illustratively includes the pump duct and the generator duct of equal sizes, one duct coupled in each circuit and disposed side-by-side with each other between the poles of a permanent magnet. The pump and generator ducts are electrically connected together by the low resistance electrodes so that a current induced by flow in one duct passes through the other duct to produce a driving pressure. The side-by-side arrangement of the two ducts are disposed between two magnetic poles, whereby an equal magnetic flux emanates through each duct. For equally sized ducts in equal magnetic fields, it has been found to be impossible to make the flow rate of the flow directed through the pump duct greater than that of the flow through the generator duct. Such a limitation is seen as a disadvantage for those reactor designs that require higher flow rates in the pump duct than in the generator duct.
The Pulley patent, noted above, discloses a flow coupler, wherein a pressure transformation between two ducts, in inverse ratio to the duct depths may be achieved. In this arrangement, the flux density in each of these ducts is maintained equal. Further, Pulley discloses that the flow velocity may be transformed in direct proportion to the duct widths. A low flow rate may be established through a secondary or pump duct of large cross-section to provide, a high flow rate in the pump duct of small cross-section, the pressure drops in the two ducts remaining equal. An examination of the Pulley patent indicates that the magnetic flux in each of the ducts of his coupler remains equal and that flow pressure or flow rate may be changed as a function of the dimensions of the ducts.